Effect Of Chromium On Microstructure And Mechanical Properties Of Carbidic Austempered Ductile Iron

Carbidic austempered ductile iron(CADI) is derived from austempered ductile iron(ADI).The mechanical properties of CADI are as excellent as that of ADI and the wear resistance property is better than that of ADI.CADI is an attractive engineering material which has better application prospect.In this paper,Microstructure observasion,Scanning electron microscopy,Spectrum analysis,X-ray diffraction analysis,rockwell hardness test,impact toughness test,impact fracture observation, wear test and wear morphology analysis were done to research the effect of chromium and heat treatment process on the microstructure,mechanical properties and wear resistance performance of CADI.The effect of chromium on the microstructure of CADI has been researched.The results showed that the microstructure of as-cast CADI containing chromium were ferrite,pearlite and carbide.With increasing of the content of chromium,the content of Pearlite was increased,content of ferrite reduced and content of carbon increased gradually.When the component not containing molybdenum and copper,the form of carbide was(Cr,Fe)₇C₃.And when the component containing molybdenum and copper,its form were(Cr,Fe)₇C₃ and(Fe,Mo)₃C.Analysing the heat treatment process perspective orthogonal result through range analysis,the order of the influence factors were:austenitic temperature＞austempering temperature＞austenitic time=austempering time.And the optimized heat treatment process is austeniting at 920℃for 1.5h and then austempering at 280℃for 2h.When the the austennitic temperature was increased,the fragmentized ferrite was decreased.While the austempered temperature was increased,austenite contents increased and the morphology of bainite from needle to the Characteristic of "feather".The effect of Chromium on the mechanics properties of CADI showed that after austeniting at 920℃for 1.5h and then austempering at 280℃for 2h,with increasing of chrome content,the hardness increased from HRC44.1 to HRC54.4,while the impact toughness reduced from 29.40J/cm² to 11.44J/cm².When autempering at 280℃,the fracture mechanism was mainly brittle fracture,which characteristics was cleavage step.Upto 320℃,the fracture mechanism were both ductile fracture and brittle fracture.While at 360℃,the fracture mechanism was ductile fracture which the fractographies of specimens changed from cleavage facets to dimple.The wear resistance rusults showed that after austeniting at 920℃for 1.5h and then austempering at 280℃for 2h,with the increase of chrome content,wear rate reduced from 2.08 mg/m to 0.82 mg/m.While austempering at 280℃,the wear surface which had no adhesive surface and metallic transfer was smooth and flat.And its wear mechanism was micro-ploughing.While at 320℃,its wear mechanism was micro-cutting and micro-spalling.But after austempering at 360℃,its wear mechanism was mainly furrow action and a little micro-cutting and micro-spalling.Through the research on heat treatment process,microstructure and properties of CADI,the optimized chromium content was 0.5wt.%.While after austeniting at 920℃for 1.5h and then austempering at 280℃for 2h,there were 16.49%retained austenite,10%carbidic and the Bainite morphology was needle in the microstructure of samples containing molybdenum and copper,which were good comprehensive properties.The hardness was HRC 50.9,impact toughness was 21.37 J/cm² and the wear rate was 1.41 mg/m.